Fans of Selena Gomez and Vanessa Hudgens be warned: Spring Breakers is not the usual brand of Sunny Delight. Inside you will find perky Gomez – Unicef goodwill ambassador and significant other to Justin Bieber – smoking a bong and talking trash. Inside you shall find winsome Hudgens – formerly of High School Musical fame – toting a pistol and inviting the patrons of her local Chicken Shack to "give me your motherfucking money or I'm going to shoot your fucking brains out". It's horrid, it's ghastly, it's bizarrely engrossing. Tween entertainment hasn't undergone so radical a makeover since Pee-Wee Herman checked into that porno theatre.

Left-field writer-director Harmony Korine's new picture is quite the weirdest, wildest beast we've seen in this year's Venice competition – a college-kid caper that's not so much a case of Korine moving to the mainstream as him showing us just how woozy and debauched the mainstream can be. Gomez stars as Faith, the God-fearing good girl who takes a vacation with her more hedonistic buddies Candy (Hudgens), Brit (Ashley Benson) and Cotty (Rachel Korine, the director's wife). Unable to fund the trip by legitimate means, the quartet elect to rob a fast-food joint and light out for Florida. "I'm starting to think this is the most spiritual place I've ever been," coos Faith in voiceover while the visuals provide a slow-motion montage of jiggling butts and copious drug use. In Korine's world, the sacred and profane have a habit of blurring.

Matters take a further swerve into the rough when the girls are first busted by the cops and then bailed out by Alien (James Franco), a jittery drug-dealer with extravagant corn-rows, silver teeth and a tattooed tear-drop below one eye – just to show how sensitive he is. Alien owns a gaudy mansion on the coast. He has guns on the wall, banknotes on his bed and a grand piano by the pool that he uses to serenade guests with tinkling covers of Britney Spears songs. "I'm the answer to your prayers," he tells the new arrivals. Incredibly, at least one of them appears to believe him.

Full credit to Korine, who sustains this act of creative vandalism right through to the finish. Spring Breakers unfolds as a fever dream of teenage kicks, a high-concept heist movie with mescal in the fuel tank. The director monkeys with the plot and kicks away the signposts. He loops the dialogue and drags a dilated, ecstatic camera-lens across honeyed flesh and painted nails. The result is his most fully realised, purely satisfying feature film since Gummo, his outrageous directing debut, way back in 1997.

In the end, it's not just the girls who return from their "break from reality" wearing a bold new set of threads. Korine went to Florida as the ageing enfant terrible of arthouse independent cinema, his career in a cul-de-sac, his future behind him. He bounces back like a man possessed, rekitted as some 21st-century Russ Meyer, playing disreputable paterfamilias to a fresh breed of supervixens. On the evidence of Spring Breakers, the role seems to suit him.